The existence of an operator< or the existence of a template specialization. Not as nice, and incapable of runtime polymorphism, but that's typically how it's d

The existence of an operator< or the existence of a template specialization. Not as nice, and incapable of runtime polymorphism, but that's typically how it's done. Concepts would have been more explicit about it but they're also compile-time only. Interfaces or pure virtual classes are not the same because they're closed.